1. Field of the Invention
The present invention relates to a suspension assembly for an automotive engine, and more particularly to a suspension assembly for an automotive engine of a front-engine and front-drive type vehicle in which automotive engines are arranged in a horizontal direction with respect to the longitudinal direction of the vehicle.
2. Description of the Related Art
In general, in an engine mounting vehicle, a sub-frame is mounted through body mounts onto a body frame. Then, an engine is supported to the body frame by engine mounts mounted on this sub-frame.
FIGS. 15 and 16 show an example of conventional general engine mounts for mounting an engine in the horizontal direction with respect to the longitudinal direction of the vehicle which is a so-called FF vehicle. In FIGS. 15 and 16, character Fr denotes a front side direction of the vehicle, character RH denotes a right hand direction of the vehicle, character UP denotes an upward direction of the vehicle and character Ge denotes a center of gravity of the engine 13. Incidentally, these characters are used in the same meaning also in the other drawings.
The sub-frame 11 is supported at its four corners through the body mounts 12 to the body frame (not shown). Then, the engine 13 is arranged in the horizontal direction in the central portion of the sub-frame 11 in the longitudinal direction. Engine mounts 14a and 14b for supporting a static load of the engine 13 are arranged respectively on a front cross member 11a and a rear cross member 11b in the vicinity of one body mount 12 of each cross members 11a and 11b. Incidentally, there are some cases where the engine mounts 14a and 14b are a left side rail 11c and a right side rail 11d of the sub-frame 11, respectively. Also, in these cases, the engine mounts 14a and 14b are arranged respectively in the vicinity of one body mount 12 of each side rail 11c and 11d.
In case the engine mounts 14a and 14b are mounted so that they are kept in the condition that they are located in the vicinity of each body mount 12 respectively, both the "idle vibration property" and the "drivability" would become worse. The "idle vibration property" represents how less the sympathetic vibration of the vehicle is caused by vibration during the idle speed of the engine. If the idle vibration property is superior, the frames hardly vibrates, whereas if the idle vibration property is inferior, the frames are liable to resonates. Also, the "drivability" means that the frames are shifted relative to the torque variation of the engine. If the drivability is inferior, the passenger is subjected to a considerably shock during the torque variation, whereas if the drivability is superior, the shock feeling would be weak.
The idle vibration property and the drivability in the arrangement of the conventional engine mounts will now be described in detail.
When the engine 13 takes an idle rotation, the engine 13 is vibrated in a rolling direction about an inertia main axis I. The inertia main axis I is hereinafter referred to as a rolling inertia main axis I. The vibration which occers at the time when the engine 13 takes the idle rotation is a so-called idle vibration. Generally the rolling inertia main axis I means that an axis, when a rigid member is revolved about the axis, in which any moment for changing the direction of the axis as viewed in a coordinate which revolves together with the rigid member is not generated and is an intrinsic axis to the engine.
By the way, the vibration mode of the body frame upon the idle vibration is a two-node bending mode as indicated by a solid line in FIG. 17. Accordingly, the body frame has a high sensitivity relative to the vibration input in the vertical direction. Incidentally, the vibration mode is depicted in the drawing in an exaggerated manner to some degree. Also, a broken line in FIG. 17 shows a skeleton of the body frame when the engine stops.
If the engine mounts 14a and 14b are mounted on the cross members 11a and 11 respectively so that they are kept in the condition that they are located in the vicinity of each body mount 12 as described above, since the engine mounts 14a and 14b are remote from the rolling inertia main axis I of the engine 13, the idle vibration is introduced into the engine mounts 14a and 14b in the vertical direction. The vertical vibration input is attenuated by the engine mounts 14a and 14b and the body mounts 12. However, although the input would be attenuated, the idle vibration is transmitted as the vertical vibration input from the engine mounts 14a and 14b to the body frame through the sub-frame 11 and the body mounts 12. Accordingly, the idle vibration property is inferior.
On the other hand, a torque repulsive force is applied to the engine 13 upon the acceleration/deceleration of the vehicle. For this reason, the engine 13 is subjected to the torque in the rolling direction about a rolling axis R (see FIG. 18). In this case, the rolling axis R means a rotary centerline of the engine which is liable to rotate by the torque repulsive force generated in the engine upon the acceleration/deceleration of the vehicle. The rotation of the engine 13 about the rolling axis R is referred to as a rolling rotation.
If, as shown by a thin line in FIG. 18, the engine 13 is rolled on the rear side of the vehicle upon the acceleration, then the rotational torque is transmitted to the engine mount 14b, and thereafter, the rotational torque is transmitted to the body frame through the sub-frame 11 and the body mounts 12 from the engine mount 14b. As a result, the vehicle 20 is sunk downwardly and simultaneously rotatively pitched toward the front side of the vehicle 20 about the center G of gravity of the vehicle 20. Thus, the vehicle is shifted as indicated by a thin line in FIG. 18. Then, a head portion 21 of the passenger in the vehicle 20 is moved from the position indicated by a solid line to the position indicated by a thin line, the passenger feels a shock.
The reason why the vehicle 20 is rotatively pitched is that a normal plane in the engine mount 14b relative to a working line of the force F applied to the engine mount 14b due to the rolling motion of the engine 13, that is, a line connecting the rolling axis R and the engine mount 14b does not pass through the gravity center G of the vehicle 20. Then, the remoter the working line (normal plane) from the gravity center G, the larger the pitching rotational force will become.
Namely, in the arrangement of the above-described conventional engine mounts 14a and 14b, the distance between the above-described working line (normal plane) and the gravity center G of the vehicle 20 is increased so that the shock feeling upon the acceleration/deceleration is very large and the drivability becomes worse.
If, as shown in FIG. 19, the working line (normal plane) of the force F applied to the engine mount 14b passes through the gravity center G of the vehicle 20, the force of the pitching rotation is not applied to the vehicle 20. Accordingly, the vehicle 20 is only sunk downwardly as shown by the thin line in FIG. 19. Therefore, the head portion 21 of the passenger is not greatly shifted, thereby decreasing a shock that the passenger might feel.
Thus, it is necessary to improve the suspension assembly of the vehicle engine and there have been proposed a variety of inventions and devices.
For example, Japanese Utility Model Application Laid-Open No. Sho. 63-65524 discloses a suspension assembly in which the engine mounts are disposed just below the rolling inertia main axis I of the engine. In this suspension assembly, the direction of the vibration input into the engine mounts upon the idle vibration is the longitudinal direction of the vehicle without directing the direction of the vibration input in the vertical direction of the vehicle, thereby improving the idle vibration property.
However, in the suspension assembly disclosed in that publication, the arrangement of the engine mounts has no sufficient degree of freedom. Also, the degree of freedom of design of the overall vehicle is restricted. Moreover, in the arrangement, the drivability is not taken into consideration.
Japanese Patent Application Laid-Open No. Hei. 8-310438 discloses a suspension assembly having a stop mechanism for suppressing the introduction of the vibration force generated by the torque repulsive force of the engine upon the acceleration/deceleration of the vehicle into the vehicle in the vertical direction so that the vibration force is introduced into the horizontal direction.
However, in the suspension assembly disclosed in that publication, although it is possible to slightly improve the drivability but the stop mechanism is very complicated and in addition, it is necessary to provide a space for the stop mechanism. Also, in this suspension device, the engine mounts are remote from the rolling inertia axis I as in the conventional examples. Namely, there is no consideration of the idle vibration property.